Minifig Scale 2: The big behind is not a good measure
A while back I posted a pic demonstrating why minifig scale is a misnomer. Today I'm posting a new picture to help you choose a scale. It relies on matching two things: height and/or width. Try this to see what I mean.
1) Go stand in front of the family car. Stick your arms out sideways and see how that compares to the bonnet width. Probably pretty similar if you're an adult with a small car. So the width of a car is about the width of your arms out. And a minifig is about 5-wide with its arms out.
2) Stand next to your car and see where it comes up to on your body. That's about where it should come up to on a minifig who is four brick heights and one plate high (around 5.5-wide in the vertical).
3) If you match one or both of these then the car you make will look about right next to a minifig. The same rule can be applied to other things.
4) And to hone in on why minifg arse scale is crap, measure the width of your arse and then measure your arm span. The ratio is way less than 2 in 5 isn't it? The minifig's arse is a very crap scaling tool.